Chemical Reactions And Equations

When fats and oils are left open, they get oxidized in the presence of air, their smell and taste change. This means the oxidation is the main cause for rancidity in fats and oil.

The substances which prevent oxidation of the food items are called antioxidants.

Example, Nitrogen is an inert gas and does not oxidize the food. Thus, it is used as antioxidants by chips manufactures.

Balanced equation: S + 6HNO₃→ H₂SO₄ + 6NO₂ + 2H₂O

Step 1: Write the unbalanced equation

S + HNO₃→ H₂SO₄ + NO₂ + H₂O

Step 2: Compare the number of atoms of reactants with the number of atoms of products.

Step 3: Now, first let us consider nitrogen atom. If we multiply 6 in the reactant (HNO₃) and product (in NO₂), we will get the equal number of atoms.

Step 4: Write the resulting equation:

S + 6HNO₃→ H₂SO₄ + 6NO₂ + H₂O

Step 5: Now check whether the equation is balanced or not by comparing the atoms

We find that the equation is not balanced yet. As the number of hydrogen and oxygen atoms are unequal on the two sides.

first balance the oxygen atom.

Step 6: If we multiply 2 in the product (in H₂O) and 6, we will get the equal number of oxygen atoms on both sides.

Step 7: Write the resulting equation:

S + 6HNO₃→ H₂SO₄ + 6NO₂ + 2H₂O

Step 8: Now check whether the equation is balanced or not by comparing the atoms.

Step 9: Write the resulting equation:

S + 6HNO₃→ H₂SO₄ + 6NO₂ + 2H₂O

We find that the equation is balanced now.

Step 10: Write down the final balanced equation:

S + 6HNO₃→ H₂SO₄ + 6NO₂ + 2H₂O

In the electrolysis of water, the gas collected at cathode is hydrogen and the gas collected at anode is oxygen. The gas which is collected in double amount is hydrogen. This is because water contains two molecules as compared to one molecule of oxygen.

The reaction that takes place in the formation of water from H₂ and O₂ is:

2H₂ + O₂→ 2H₂O

Respiration and decomposition processes are exothermic process because:

i. We all get energy from the food we eat.

ii. During digestion, food is broken down into simpler substances.

iii. For example, Carbohydrates (rice, potatoes, bread) are broken down to form glucose.

iv. This glucose combines with oxygen and provides energy to the whole body.

O₂ is an oxidizing agent and Mg is reducing agent

2Mg + O₂→ 2MgO

In the given reaction, O₂ is reduced by losing oxygen atoms. Thus, O₂ is an oxidizing agent as it undergoes reduction.

Mg is oxidized by gaining oxygen atoms. Thus, Mg is a reducing agent as it undergoes oxidation.

Potato chips manufacturers fill the packet of chips with nitrogen gas to prevent the chips from getting oxidized. If the oxidation takes place, the chips can become rancid and their smell and taste will change.

Hence, to prevent oxidation, the packet of chips are filled with nitrogen gas. Nitrogen gas is an antioxidant.

The type of reaction is displacement reaction

In the given reaction:

Fe + CuSO₄(aq) → FeSO₄(aq) + Cu(s)

The iron metal being more reactive than copper displaces it, and forms a new compound that is FeSO₄

Balanced equation: 2Al + 3CuCl₂→ 2AlCl₃ + 3Cu

Step 1: Write the unbalanced equation

Al + CuCl₂→ AlCl₃ + Cu

Step 2: Compare the number of atoms of reactants with the number of atoms of products.

Step 3: Now, first we consider the element having unequal no. of atoms on both sides. Thus, first let us consider the chlorine atom. If we multiply 3 in the reactant (in CuCl₂) and 2 in the product (AlCl₃), we will get the equal number of atoms in both sides.

Step 4: Write the resulting equation:

Al + 3CuCl₂→ 2AlCl₃ + Cu

Step 5: Now check whether the equation is balanced or not by comparing the atoms

We find that the equation is not balanced yet. As the number of Cu and Al atoms are unequal on the two sides. First balance the Al atom.

Step 6: If we multiply 2 in the reactant (in Al), we will get the equal number of Al atoms on both sides.

Step 7: Write the resulting equation:

2Al + 3CuCl₂→ 2AlCl₃ + Cu

Step 8: Now check whether the equation is balanced or not by comparing the atoms.

Step 9: Write the resulting equation:

2Al + 3CuCl₂→ 2AlCl₃ + Cu

We find that the equation is not balanced yet. As the number of copper atoms is unequal on the two sides.

Step 10: If we multiply 3 in the product (in Cu), we will get the equal number of atoms as in reactant (in CuCl₂)

Step 11: Write the resulting equation:

2Al + 3CuCl₂→ 2AlCl₃ + 3Cu

We find that the equation is balanced now.

Step 12: Write down the final balanced equation:

2Al + 3CuCl₂→ 2AlCl₃ + 3Cu

When quicklime (calcium oxide- CaO) is added to water, it reacts vigorously with water to produce slaked lime (calcium

hydroxide) and releases a large amount of heat.

The reaction that takes place is:

CaO + H₂O → Ca(OH)₂

Quick lime Slaked lime

The above reaction is an example of combination reaction in which calcium oxide and water combined together to form a single

product, calcium hydroxide.

When HCl is diluted with copper oxide, the following reaction takes place:

HCl + CuO → CuCl₂ + H₂O

The new compound formed is copper chloride (CuCl₂) which imparts a blue-green colour to the solution. The above reaction is a type of double displacement reaction.

(a)When copper powder is heated in the presence of air (oxygen), copper reacts with oxygen to form copper oxide. The copper

oxide formed is black in colour. The black colour is formed due to the oxidation of copper takes place.

The chemical equation of the reaction that takes place is given below:

2Cu + O₂→ 2CuO

Copper oxide

(b) When hydrogen gas is passed over hot copper oxide, the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

The reaction that takes place is:

CuO + H₂ → Cu + H₂O

The chemical phenomenon which is responsible is corrosion.

When a metal is attacked by substances around it such as moisture, acids, etc. it is said to corrode and this process is called corrosion.

After corrosion, the colour of:

Silver changes to → black

Copper changes to → green

Hence, the chemical name of coatings are silver coating and copper coating.

Differences between oxidation and reduction are:

Differences between combination reaction and decomposition reaction are:

Difference between displacement and double displacement reactions:

(i) When potassium iodide is added to aqueous lead nitrate, it forms potassium nitrate and a precipitate of lead iodide which is yellow in colour.

The reaction that takes place is:

Pb(NO₃)₂ + 2KI → 2PbI + 2KNO₃

Note: The insoluble substance formed is known as precipitate.

Precipitate settles down at the bottom of the test tube.

(ii) The type of reaction is double displacement reaction or precipitation reaction.

Any reaction that produces a produces a precipitate can be called a precipitation reaction.

(iii) The balanced chemical equation is:

Pb(NO₃)₂ + 2KI → 2PbI + 2KNO₃

Oxidizing agent: An oxidizing agent is an element that gains electrons. Since the oxidizing agent means to gain electrons; it is

said to have been reduced.

The element which undergoes reduction (gets reduced) is called oxidizing agent.

For example: 2Mg + O₂→ 2MgO

In the given reaction, O₂ is reduced by losing oxygen atoms. Thus, O₂ is an oxidizing agent as it undergoes reduction.

Reducing agent: A reducing agent is an element that loses electrons. The reducing agent means to lose electrons; it is said to

have been oxidized.

The element which undergoes oxidation (gets oxidized) is called reducing agent.

For example:

4NH₃ + 5O₂→ 4NO + 6H₂O

In the given reaction, nitrogen is oxidized to NO by gaining oxygen atom. Thus, NH₃ is reducing agent as it undergoes oxidation.

When fats and oils are oxidized in the presence of air, they become rancid and their smell and taste change. This phenomenon is called rancidity.

Rancidity can be prevented by:

i. Using antioxidants- substances which prevent oxidation are called antioxidants. They are added to foods containing fats and oil.

For example, Nitrogen gas is filled in the packets of chips to prevent them from oxidation.

ii. Keeping food in air tight containers to slow down the process of oxidation.

(a) Magnesium ribbon should be cleaned before burning at air because:

i. Magnesium is a very reactive metal.

ii. When it is stored, it forms a layer of magnesium oxide (MgO) by reacting with oxygen.

iii. This layer of MgO stops the further reaction of magnesium with oxygen.

iv. Thus, magnesium ribbon should be cleaned to before burning to air to remove this layer

(b) Iron should be protected from the moist air because:

i. When iron (a metal) is attacked by moist air (moisture), it is said to corrode.

ii. This process is called corrosion or rusting of iron.

iii. Rusting of iron causes damage to car bodies, bridges, iron railings to all objects made of iron.

iv. Every year a big amount of money is spent to replace damaged iron.

v. Hence, we should protect iron from moist air.

(i) When lead nitrate [Pb(NO₃)₂] is heated, it forms lead oxide. The emission of brown fumes is also observed. These brown fumes are of nitrogen dioxide (NO₂).

The reaction that takes place is:

Pb(NO₃)₂ + heat → 2PbO + 4NO₂ + O₂

Lead nitrate lead Nitrogen Oxide dioxide

The above reaction is an example of decomposition reaction in which single reactant breaks down to give simpler products.

(ii) When green crystals of ferrous sulphate [FeSO₄] are heated, the crystals lose water and the colour of the crystals changes. It then decomposes to ferric oxide [Fe₂O₃], sulphur dioxide, and Sulphur dioxide.

The reaction that takes place is:

FeSO₄ + heat → Fe₂O₃ + SO₂ + SO₃

Ferrous sulphate sulphur Sulphur dioxide trioxide

The above reaction is also an example of decomposition reaction.

The reaction in which there is an exchange of ions between the reactant takes place is called double displacement reaction.

For examples:

Na₂SO₄ + BaCl₂→ BaSO₄ + 2NaCl

Sodium Barium Barium Sodium

sulphate chloride sulphate chloride

AgNO₃ + NaCl → AgCl + NaNO₃

Silver Sodium Silver Sodium

nitrate chloride chloride nitrate

The reactions in which oxidation and reduction are taking place at the same time are called redox reactions.

For examples:

In the above example, Fe₂O₃ is reduced to Fe by losing oxygen atoms.

Thus, Fe undergoes reduction.

CO is oxidized to CO₂ by gaining oxygen atom. Thus, C undergoes oxidation.

CuO + H₂→ Cu + H₂O

In the given reaction, CuO is reduced to Cu by losing oxygen. Thus, Cu undergoes reduction.

H₂ is oxidized to H₂O by gaining oxygen atom. Thus, H₂ undergoes oxidation.

Combination reaction is a reaction in which two or more reactants combined together to form a single product.

For examples:

When nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas. The following reaction

takes place:

Formation of water from H₂ and O₂:

2H₂ + O₂→ 2H₂O

2Na(s) + Cl₂(g) → 2NaCl(s)

In the above reaction, sodium (Na) loses electrons and produces Na⁺ Ions and thus Na acts as a reducing agent as it undergoes oxidation. On the other hand, chlorine gains electrons and forms chloride ions and thus chlorine acts as an oxidizing agent as it undergoes reduction.

White silver chloride turns grey in sunlight due to the decomposition of silver chloride into silver and chlorine by light.

The reaction that takes place is:

2AgCl + sunlight → 2Ag + Cl₂

The above reaction is an endothermic reaction.

Iron nails when left dipped in blue copper sulphate solution become brownish in colour and the blue colour of copper sulphate fades away.

Fe + CuSO₄→ FeSO₄ + Cu

The above reaction is a type of displacement reaction in which one element displaces another element from its compound and takes its place therein. In the reaction, Fe replaces Cu and form a new compound.

When limewater (CaOH)₂ is passed through CO₂, the following reaction takes place:

(CaOH)₂ + CO₂→ CaCO₃ + H₂O

Limewater Calcium

Carbonate

i. In this reaction, when limewater comes in contact with the gas released in the form of an effervescence, it turns milky. This

chemical test for carbon dioxide gas.

ii. When limewater turns milky, it is confirmed that the effervescence is of carbon dioxide.

When calcium oxide- CaO is added to water, it reacts vigorously with water to produce slaked lime (calcium hydroxide) and releases a large amount of heat.

The reaction that takes place is:

CaO + H₂O → Ca(OH)₂

Quick lime Slaked lime

In the given reaction,

3MnO₂(s) + 4Al(s) → 3Mn(ℓ) + 2Al₂O₅(s) + Heat

Aluminum being more reactive displaces the manganese metal and forms a new compound. Hence it is a displacement reaction.

The heat is also released along with the formation of products, hence it is an exothermic reaction.

HCl is oxidized by gaining oxygen atoms. Thus, HCl is a reducing agent as it undergoes oxidation.

CuO is reduced by losing oxygen atoms. Thus, CuO is a reducing agent as it undergoes reduction.

When a metal is attacked by substances around it such as moisture, acids, etc. it is said to corrode and this process is called corrosion.

After corrosion, the colour of:

Iron changes to → Reddish brown

Silver changes to → black

Copper changes to → green

When fats and oils are oxidized, they become rancid and their smell and taste change. This phenomenon is called

rancidity. Basically, oxidation causes rancidity.

Nitrogen is an antioxidant which prevents oxidation. By keeping food in airtight containers by adding nitrogen helps to slow oxidation.

By keeping the fat/oil containing food materials in an open area or by adding oxygen can cause rancidity.

The colour formed on the surface of copper powder when it is heated in a china dish is black. Oxidation of copper takes place. As a result, copper oxide is formed which is black in colour.

When hydrogen gas is passed over hot copper oxide, the black coating on the surface turns brown as the reverse reaction takes place and copper is obtained.

2AgBr(s) + sunlight → 2Ag(s) + Br₂(g)

The above reaction is a type of decomposition reaction which is used in black and white photography.

4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g)

The above reaction is a type of displacement reaction in which one element displaces other element from its compound.

It is also a redox reaction in which oxidation and reduction of a substance take place at the same time.

3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

In the above reaction, the iron metal is getting oxidized to Fe₃O₄ by gaining oxygen atoms. H₂O is getting reduced to H₂ by losing oxygen atom. Hence, water is an oxidizing agent as it undergoes reduction.

Quicklime reacts vigorously with water and releases a large amount of heat along with the formation of products. Hence it is an exothermic process. Dilution of acid is also an exothermic process.

Potassium permanganate (KMNO₄) is a strong oxidizing agent. It oxidizes FeSO₄.

The reaction (i) is a type of displacement reaction as Pb displaces Cu and form a new compound PbCl₂

The reaction (ii) is a type of double displacement reaction in which exchange of ions takes place.

The reaction (iii) is a type of combination reaction in which carbon and oxygen are combined together to form a single product that is CO₂.

The reaction (iv) is a type of exothermic reaction as the burning of

natural gas (CH₄) releases a large amount of energy.

White silver chloride turns grey in sunlight due to the decomposition of silver chloride into silver and chlorine by light.

The reaction that takes place is:

2AgCl + sunlight → 2Ag + Cl₂

Reaction in which energy is absorbed is known as endothermic reaction. Hence, sublimation of dry ice and evaporation of water are endothermic reactions as they both involve adsorption of heat.

If lead nitrate is not available, we can use lead acetate in place of it. We cannot use lead sulphate because it is insoluble. In ammonium nitrate and potassium sulphate, the ions are different. We can only use the compound of Pb not any other metal.

In reaction (c), H₂ and O₂ are in gaseous state and the product formed that is water is in liquid state. This represents the correct state of the reactants and products.

In reaction (ii), MgO + H₂O → Mg(OH)₂

MgO and water combined together to form a single product, i.e., magnesium hydroxide.

In reaction (iii), 4Al + 3O₂→ 2Al₂O₃

Aluminium combines with oxygen to form a single product. i.e., aluminum oxide

Hence, both are examples of combination reactions.

False

When green crystals of ferrous sulphate (FeSO₄) are heated, the crystals lose water and the colour of the crystals changes to brown. This is decomposition reaction.

Hence, the give statement is false.

True

Calcium oxide (CaO) is also called quicklime. Quick lime is formed when calcium carbonate (limestone) is heated. This is also an example of decomposition reaction.

Hence, the give statement is true.

True

When crystals of lead nitrate [Pb(NO₃)₂] is heated, it forms lead oxide. The emission of brown fumes is observed. These brown fumes are of nitrogen dioxide (NO₂).

Hence, the give statement is true.

False

The thermal decomposition reaction of calcium sulphate (gypsum) is not used in black and white photography. The

reaction of silver bromide with sunlight to for silver and bromine used in black and white photography.

Hence, the give statement is false.

False

The decomposition reaction of silver bromide into silver and bromine by light is used in black and white photography.

The reaction that takes place is:

2AgBr + sunlight → 2Ag + Br₂

Hence, the give statement is false.

True

The insoluble substance formed during a chemical reaction is known as a precipitate. The precipitate formed in a double displacement reaction.

Hence, the given statement is true.

False

During endothermic reactions, heat is transferred from the surroundings to the reacting substances. On the other hand, heat is transferred from the reacting substances to the surroundings.

Hence, the given statement is false.

False

The reaction is an example of displacement reaction in which zinc metal displaces other metal, i.e., copper from CuSO₄ to form a new compound ZnSO₄.

Hence, the given reaction is false.

True

Keeping food in airtight containers helps to slow down oxidation. Airtight containers prevent the food materials from

getting oxidized.

Hence, the given statement is true.

True

When a metal is attacked by substances around it such as moisture, acids, etc. it is said to corrode and this process is called corrosion.

After corrosion, the colour of:

Iron changes to → Reddish brown

Silver changes to → black

Copper changes to → green

Hence, the given statement is true.